1. Field of the Invention
This invention relates generally to the electrode structure and manufacture processes of rechargeable batteries. More particularly, this invention relates to an improved structure and manufacture process of battery electrode to provide battery with better performance while reducing the battery internal pressure and providing a more streamline manufacture processes to produce batteries at lower cost.
2. Description of the Prior Art
High internal pressure in the sealed battery cells often creates a hazardous conditions, when the pressure exceeds certain thresholds, which becomes a severe product liability concern for battery manufactures. Especially, since the sealed battery cells are now being widely used for every conceivable consumer products, to assure that the internal cell pressure would not exceed a limit in its entire life becomes a public safety issue. However, in the design and manufacture of different types of batteries, the internal pressure caused by gases produced by chemical reactions in the discharge or charge cycles still is a major technical difficulty. The inability and lack of effective solution to this problem often limits the product yield and unnecessarily increases the production costs of batteries.
Matsumoto et al. disclose in U.S. Pat. No. 4,251,603 entitled `Battery Electrode` (Issued on Feb. 17, 1981), a battery electrode includes a plaque made of a sponge-like porous metal matrix having a multiplicity of cells connected with each other three-dimensionally. The sectional area of the gratings making up the sponge-like metal porous plaque decreases continuously along the thickness of the plaque from the surface toward the central part and an active material is impregnated in the porous plaque. The object of the patented invention is to provide a battery electrode comprising a plaque of a sponge-like porous metal matrix with high density of an active material impregnated in the plaque and the plaque can be produced at a low cost. Matsumoto et al. disclose a fundamental method of making battery electrodes. The basic technique as disclosed however does not provide an electrode to reduce the gas-pressure generated from chemical reactions with the battery cells.
Dinker et al. disclose in U.S. Pat. No. 4,460,666, entitled "Coated Substrate, Preparation Thereof, and Use Thereof" (issued on Jul. 17, 1984), a conductive substrate with major surfaces embossed and with at least on of the surfaces processed with a sintered porous metal powder coating. The special embossed surface increases the sintering speed, provides better handling in electrochemical cleaning and impregnation, and strengthens the adhesion of the active material to the surfaces of the substrate thus improves the integrity of the electrode. The difficulty of high cell gas pressure are not resolved by the use of special electrodes formed with such coated substrate.
Kober et al. disclose in U.S. Pat. No. 4,707,911, entitled "Porous Electrodes and Method of Making Same" (issued on Nov. 24, 1987), porous electrodes for lead-add storage batteries without supporting plates or grids. Improved performance characteristics are achieved because these batteries are lighter in weight, having minimum internal resistance, providing higher rate of discharge, and more resistant to corrosion. The use of foams, metal nets, porous substrate, paste type of electrodes, and the fabrication methods of making those electrodes are disclosed in patents such as: U.S. Pat. Nos. 4,687,553, 5,455,125, 5,434,023, 5,434,019, 5,432,031, 5,405,719, 5,384,216, 5,374,491, 5,348,823, 5,329,681, 5,324,333, 5,248,510, 5,244,758, 5,098,544, 5,077,149, 4,978,431, 4,975,230, 4,957,543, and 4,929,520. These disclosures provides a broad spectrum of techniques of making improved electrodes and batteries. Yet, none of them provides a solution to the technical difficulties of high gas cell pressure in a sealed battery.
In order to reduce internal pressure, `auxiliary electrodes` are employed. As disclosed in Section 13.3.3 in `Battery Design` (See "Maintenance-Free Batteries-A Handbook of Battery Technology" by Brant, published by Research Study Press, Ltd. in year 1993) a prismatic sealed nickel-cadmium battery with auxiliary electrode is shown as that shown in FIG. 1. The negative (Cd) electrode is split into two plates, with an untilled nickel fiber electrode (A) interposed between the adjacent cadmium electrode. The negative electrodes are separated from a positive electrode (Ni) by spacers (S). The untilled plaque, i.e., the auxiliary electrode A, acts as a catalytic site for rapid oxygen reduction. The arrow indicated the main pathway for the oxygen in the gas phase to reach the untilled nickel substrate. As shown in FIG. 1, auxiliary electrode A which is composed by nickel fiber without impregnation are covered on both sides by the cadmium (Cd), i.e., the active hydrogen storage material for the negative electrode.
The basic configuration as that shown in FIG. 1 becomes a typical electrode configuration when nickel mesh or sponge-like porous metal matrix are used in forming the negative electrode with the nickel fiber serves as auxiliary electrode. Please refer to FIG. 2 for the structure of a conventional electrode 10. This conventional electrode 10 includes a three layer structure, i.e., a first active layer 20 and a second active layer 30 formed on two opposite sides of a carrier layer 40. The active layers 20 and 30 are generally formed with hydrogen storage alloys and the carrier layer 40 is typically formed with nickel mesh or sponge-like porous metal matrix. Such structure is commonly employed in the electrodes as disclosed in the above prior art patents. The difficulty of high internal cell pressure is not resolved by this type of electrode construction.
FIG. 3 shows an improved electrode 50 from the conventional electrode as shown in FIG. 2. The electrode 50 includes two primary electrodes consisting of a three layer structures, i.e., first primary electrode includes layers 60, 70, and 80, and second primary electrode includes layers 70, 80, and 90. These two primary electrodes are disposed on both sides, with an auxiliary electrode 95 disposed between them. The auxiliary electrode 95 provides a catalyst section to reduce the internal pressure. This configuration had the disadvantage that the manufacture of such an electrode is more complicate and more costly, and the surface areas provided for by the auxiliary electrode 95 is not sufficient to assure low internal pressure can be consistently achieved.
Therefore, a need still exists in the art of design and manufacture of battery electrodes to provide an improved structural configuration and method of fabrication of electrodes. The improved electrodes must be effective to reduce the internal gas pressure and is simple to fabricate such that the time and cost of manufacture can be reduced.